feat(api/cert): enhance certificate info retrieval

- Introduced a new method `GetCertInfos` to fetch details of all available certificates.
- Updated the `Info` handler to return an array of `CertInfo` instead of a single certificate.
- Improved error handling for cases with no available certificates.
- Refactored related error messages for clarity.

internal/api/v1/docs/swagger.yaml

@@ -1886,17 +1886,19 @@ paths:
         "200":
           description: OK
           schema:
-            $ref: '#/definitions/CertInfo'
+            items:
+              $ref: '#/definitions/CertInfo'
+            type: array
         "403":
-          description: Forbidden
+          description: Unauthorized
           schema:
             $ref: '#/definitions/ErrorResponse'
         "404":
-          description: Not Found
+          description: No certificates found or autocert is not enabled
           schema:
             $ref: '#/definitions/ErrorResponse'
         "500":
-          description: Internal Server Error
+          description: Internal server error
           schema:
             $ref: '#/definitions/ErrorResponse'
       summary: Get cert info